1. Field of the Invention
The present invention relates to a display device and a driving method thereof, and in particular, to a hold-type display device and a driving method thereof.
2. Description of the Related Art
In a hold-type display device such as a liquid crystal display or an organic electro-luminescent (EL) display, unlike an impulse-type display device such as a cathode ray tube (CRT) display, an image is continuously held during a frame period, which makes a moving image unclear. More specifically, in the case of the hold-type display device, an image is held and displayed as a still image in a frame period, and the moving image is displayed by switching the screen for every frame. Therefore, the still image is seamlessly switched in a transition period from one frame to another. This causes the user to recognize the frame image of one before as an after image, to sense a double image where the images of both frames are overlapped, and to recognize a moving image blur.
As a method for resolving the moving image blur, there is a proposed technique for reducing the moving image blur caused due to overlapping recognition of the current frame image and the afterimage of the frame one before by inserting a black image in a single frame period at a predetermined rate, and the technique is practically used.
Here, references are made to the technique for inserting a black image in a single frame period at a predetermined rate.
FIG. 9 is a graph showing a black insertion technique. The horizontal axis represents video signal data. The vertical axis represents luminance level in a single frame. The video signal data represented by the horizontal axis is, for example, a digital value that is an input gray level ranging from 0 to 255. In FIG. 9, in the case where the black insertion rate is 0%, in other words, in the case where black insertion is not performed in a single frame period, 0 to 255 that is the input gray level of the video signal data correspond to the luminance level of 0 to 100% in a single frame. On the other hand, for example, in the case where a black image is inserted in 40% of a single frame period, in other words, in the case where the black insertion rate is 40%, 0 to 255 that is the input gray level of the video signal data correspond to the luminance level of 0 to 60% in a single frame. Further, for example, in the case where a black image is inserted in 80% of a single frame period, in other words, in the case where the black insertion rate is 80%, 0 to 255 that is the input gray level of the video signal data correspond to the luminance level of 0 to 20% in a single frame. More specifically, when a user recognizes the luminance of a single frame, not only the input gray level of 0 to 255 is changed for light-emitting display, but also the black insertion rate is changed even in the same input gray level. This enables variation of the luminance level to be recognized. For example, the luminance level of a single frame that is recognized by the user is the same between the case where light is emitted in 50% of a single frame period at the input gray level of 255 and the black image is inserted in the remaining 50% at the gray level of 0, and the case where light is emitted in the entire single frame period at the input gray level of 127. However, the former case where the black image is inserted between frames can reduce the moving image blur.
Japanese Patent Application Publication No. 2006-018138 discloses a black insertion technique where, in an optically compensated birefringence (OCB) type liquid crystal display device, a high voltage is periodically applied not only during a blanking period, but also during a video signal writing period to prevent occurrence of the inverse transfer that is caused if the driving voltage is maintained at low voltage level. According to the disclosure, it is possible to set a high voltage period for preventing occurrence of the inverse transfer over a long period.
Japanese Patent Application Publication No. 2008-268886 discloses a technique where a black image is inserted at a predetermined rate in a single frame period to enhance the image quality of the moving image and where the black insertion rate with respect to one frame period is set variably according to each usage state.